The present invention relates generally to electrical contact materials and assemblies, and more particularly to a method of producing a welded contact assembly having a nonwelding electrical contact surface and to a composite material for producing such a contact assembly.
It is necessary and well known in connection with electrical switches for high current and high voltage applications to use a contact material which resists welding to prevent fusing of electrical contacts due to arcing upon breaking and/or making of the contacts. However, the same properties which make a material suitable for nonwelding contacts also limit the assembly and fabrication processes which can be used. With such materials, heat based bonding methods such as soldering, brazing or welding are difficult to use, and adhesion of the contact to the contact carrier may not be structurally sound. Further, the electrical properties of such an assembly may be adversely affected. Yet further, contact materials made of silver and metal oxide composites, which have highly desirable nonwelding properties, cannot be practically welded by resistance welding methods. This is a distinct disadvantage because resistance welding is one of the most inexpensive, simple and reliable methods of attaching a contact to a contact carrier.
A variety of techniques have been attempted to permit the use of welding in attaching contacts having nonwelding characteristics to contact carriers. One approach has been to form a layer of a metal oxide on a base material having good welding properties. The base material can then be welded to the contact carrier and the oxide layer on the base material forms the electrical contact surface. Another approach has been to form or bond a layer of material having good welding characteristics on a nonwelding material which provides the electrical contact surface. For example, U.S. Pat. No. 2,425,053 issued to M. Swinehart on Aug. 5, 1947 and U.S. Pat. No. 2,468,888 issued to H. Mekelburg on May 3, 1949 each disclose electrical contacts which are individually formed by placing a layer of silver or silver alloy powder in a suitable die cavity, that layer then being covered with a layer of a suitable metal oxide powder. Thereafter the powder in the cavity is subjected to a high pressure molding operation and heat sintering. The resulting contact has a nonwelding metal oxide electrical contact surface and a metal backing which exhibits good welding properties. A disadvantage is that this process, in which the contacts are individually formed, is relatively slow and expensive.
Another technique is shown in U.S. Pat. No. 4,342,893 issued to H. Wolf on Aug. 3, 1982. In this technique, a ribbon of composite contact material is formed by a rolling process in which a wire of a metal oxide is rolled together with one or more wires of a metal such as a silver copper alloy solder to form a tape material having a nonwelding electrical contact surface and one or more beads of a material with good welding properties on the opposite surface for permitting welding of segments of the tape to a contact carrier. One of the disadvantages of a rolling operating is that it cannot be conducted at the temperature sufficiently high to achieve a metallurgical bond between the metal alloy and metal oxide materials. For present purposes, a metallurgical bond is defined to be a bond in which there is significant diffusion of the two materials into one another at their interface. A metallurgical bond between the metal and metal oxide materials is desirable and/or necessary in order to achieve required structural properties of the composite contact material and of the contact/contact carrier assembly. In the technique described in the previously identified Wolf patent, if a sufficiently high temperature for achieving a metallurgical bond is used, the metal and/or metal oxide materials would tend to adhere to the forming rollers.
The applicant has avoided the foregoing problems by providing a hot extruded composite contact material and method of producing electrical switch contact assemblies in which a true metallurgical bond is formed between the nonwelding metal oxide material and a metal layer having good welding characteristics. The composite contact material is economically producible in wire form and suitable for use in highly integrated automatic switch assembly processes and machines.